1. Field of the Invention
This invention relates to a method for the production of an aqueous quaternary ammonium hydroxide solution of extremely high purity to be used as a treating agent for the washing of semiconductor substrates (wafers) and the development of resist films in the process for the manufacture of semiconductor devices.
2. Description of the Prior Art
In the electronic industry, the aqueous quaternary ammonium hydroxide solution has found extensive utility as a treating agent for the washing and etching of wafers and the development of resist films formed on the cleaned wafers in the process for the manufacture of IC's and LSI's.
If the aqueous quaternary ammonium hydroxide solution so used as the treating agent contains impurities, the impurities remain on the wafers or on the resist films after the work of washing or development and eventually bring about such adverse effects as impairing the precision of IC's and LSI's to be produced or interfering with the operation of electronic circuits. In the storage of the aqueous quaternary ammonium hydroxide solution, there have been times when highly corrosive impurities persisting in the aqueous solution corrode the wall of the container and induce further decline of the purity of the solution.
The contents of impurities in the aqueous quaternary ammonium hydroxide solution, therefore, are rigidly controlled. In U.S. Pat. No. 4,394,226, for example, such cations as Li, Na, K, Fe, Ni, Al, Cr, and Zn are defined not to exceed 10 ppm and such anions as Cl, Br, and I not to exceed 4,000 ppm. In fact, the contents of these impurities are desired to be as low as possible. The desirability of removing such impurities from the aqueous solution to the fullest possible extent has found growing recognition. Particularly, in consequence of the recent notably increasing integration of circuits in semiconductor devices, there has appeared a growing demand for the aqueous quaternary ammonium hydroxide solution which enjoys high purity, excels in stability of storage, and refrains from inducing occurrence of the harmful impurities mentioned above during storage.
For the production of the conventional aqueous quaternary ammonium hydroxide solution, there have been proposed various methods such as are cited below, for example.
One method effects the production of an aqueous quaternary ammonium hydroxide solution by the reaction of an aqueous solution of a halide salt of quaternary ammonium with an aqueous suspension of silver oxide and another method effects the production of an aqueous quaternary ammonium hydroxide solution by the steps of causing chloride of quaternary ammonium to react with potassium hydroxide in methanol and removing the precipitated potassium chloride from the methanol solution by filtration. The first method suffers from inclusion of silver ion in the final product and the latter method suffers from inclusion of chloride and potassium in the final product. Thus, the two species of aqueous quaternary ammonium hydroxide solution obtained by these methods are not applicable to the production of electronic circuits of high quality.
Still another method which is disclosed in U.S. Pat. No. 3,523,068 effects the production of an aqueous quaternary ammonium hydroxide solution by electrolyzing a salt of quaternary ammonium possessing a non-electrolytic anion in an electrolytic cell having an anode and a cathode separated from each other with an intervening cation-exchange membrane. As concrete examples of the non-electrolytic anion, nitrate radical, carbonate radical, and sulfate radical are cited. The specification of this U.S. patent is found to include a statement to the effect that the sulfate radical is particularly desirable as the non-electrolytic anion. In the working examples actually cited, nothing is mentioned of any non-electrolytic anion other than the sulfate radical. Moreover, the aqueous solution produced by this method is intended as an electrolyte or as a pH adjusting agent. Thus, the purity of the product does not matter very much. The significance of purity of the produced aqueous quaternary ammonium hydroxide solution is mentioned nowhere in the specification.
Yet another method intended for the production under discussion is disclosed in U.S. Pat. No. 4,394,226. This method effects the production of an aqueous quaternary ammonium hydroxide of high purity by electrolyzing a halide of quaternary ammonium in an electrolytic cell having an anode and a cathode separated from each other with an intervening cation-exchange membrane. Even this method poses various problems as shown below.
During the course of electrolysis, the anolyte produces harmful and highly corrosive halogen ion and halogen gas both in high concentration, the anode itself which is made of a metal such as platinum is corroded by the halogen ion, for example, and the product of this corrosion passes through the ion-exchange membrane into the catholyte, with the result that the purity of the produced aqueous quaternary ammonium solution will be eventually degraded. Further, the solution obtained at the end of the electrolysis is required to be treated for removal of harmful materials therein, the cost for the disposal of the effluent is consequently increased.
There is another problem that the halogen ion mentioned above deteriorates the anode compartment and the cation-exchange membrane, both made of synthetic resins, during the course of electrolysis. Because of this problem, the ion-exchange membrane made of polystyrene, for example, cannot be used at all in the electrolysis in this case. Even when the cation-exchange membrane made of a fluoro-carbon type resin which excels in durability is used, the aforementioned deterioration is so serious that this membrane will not withstand protracted service. Thus, the membrane does not suit commercial operation of the method.
Moreover, the ion selectivity and the gas barrier property of the cation-exchange membrane are not perfect. The method, therefore, has the disadvantage that the halogen ion and the halogen gas pass through the exchange membrane, both in minute amounts, and mingle into the catholyte, i.e. quaternary ammonium hydroxide. When the aqueous quaternary ammonium hydroxide solution so produced is stored in a generally adopted container of stainless steel, therefore, the highly corrosive halogen ion persisting in the aqueous quaternary ammonium hydroxide solution corrodes the stainless steel container and consequently brings about a decline in the purity of the aqueous solution while in storage.
To overcome the drawbacks suffered by the conventional methods, the inventor already proposed in U.S. patent application No. 665,524 a method for producing an aqueous quaternary ammonium hydroxide solution by synthesizing the formate of quaternary ammonium by the reaction of a trialkylamine with a formic ester and subsequently electrolyzing the formate in an electrolytic cell using a cation-exchange membrane as a diaphragm.
Since this method is capable of avoiding occurrence of a halogen ion in a high concentration in the anolyte during the electrolysis, the otherwise possible deterioration of the cation-exchange membrane can be prevented and the aqueous quaternary ammonium hydroxide solution can be produced in high purity with an improved shelf life.
The inventor, as the result of a continued study, has found that the aqueous quaternary ammonium hydroxide solution obtained by this method contains a minute amount of formate ion (HCOO.sup.-) and the formate ion corrodes the stainless steel container and, consequently, the aqueous solution loses slightly in purity while the aqueous solution is kept in storage and that when aqueous quaternary ammonium hydroxide solution of this nature is used as a developing agent, the formate mentioned above has affinity for the resist film of an organic substance and, therefore, produces a notable effect upon the developing property of the aqueous solution, and that there arises the possibility of the uniformity of the developing condition and that of the quality of the final product being impaired by the variation in the formate content of the aqueous solution.
This method has a further problem in that during the electrolysis of the formate of quaternary ammonium, the amount of electricity required is twice as large as when the aforementioned halide of quaternary ammonium is electrolyzed because formic acid is produced on the anode and this formic acid is electrolyzed and oxidized to the level of carbon dioxide gas and, during the synthesis of the formate of quaternary ammonium by the reaction of a trialkylamine with a formic ester, since the formate requires the presence of high temperature (about 130.degree. C.) and high pressure (about 20 kg/cm.sup.2), it undergoes partial decomposition to a point where the conversion itself is lowered and the yield of the final product of quaternary ammonium hydroxide is consequently lowered.